The invention herein described relates generally to a radiation detector assembly and a method for assembling the same. The radiation detector assembly is particularly useful for well bore hole logging applications, but may also have use in other applications.
Radiation detectors have been employed in the oil and gas industry for well logging. These detectors have used thallium-activated sodium iodide crystals that are effective in detecting gamma rays. The crystals are enclosed in tubes or casings to form a crystal package. The crystal package has an optical window at one end of the casing which permits radiation-induced scintillation light to pass out of the crystal package for measurement by a light sensing device, such as a photomultiplier tube (PMT) coupled to the crystal package. The PMT converts the light photons emitted from the crystal into electrical pulses that are shaped and digitized by associated electronics. Pulses that exceed a threshold level are registered as counts that may be transmitted xe2x80x9cupholexe2x80x9d by wire to analyzing equipment or stored locally.
The ability to detect gamma rays makes it possible to analyze rock strata surrounding the bore holes, as by measuring the gamma rays coming from naturally occurring radioisotopes in down-hole shales which bound hydrocarbon reservoirs. Today, a common practice is to make measurements while drilling (MWD). For MWD applications, the detector must be capable of withstanding high temperatures and also must have high shock resistance. At the same time, there is a need to maintain performance specifications.
In a typical integrated detector assembly, a crystal package, a photomultiplier tube, and a voltage divider are enclosed in a housing. Some view the PMT as the weakest link in the assembly and have required the PMT to be field-replaceable. In the past, this field replacement capability has been accomplished by using a threaded shield cap on the back side of a PMT casing. The cap can be removed to permit removal and replacement of the PMT.
However, it is important that the integrity of the detector assembly be preserved after a field replacement. Among other things, proper loading of the PMT is important from a shock and vibration standpoint, especially in MWD applications. The PMT needs to be loaded in such a manner both radially and axially in its housing so that it does not move under certain vibration responses. If it does move, static discharges may be created which may be mistaken for light events emanating from the crystal, thereby causing inaccurate readings.
From the foregoing it can be seen that it would be desirable to have a radiation detector assembly that can be disassembled in the field for replacement of a PMT, and yet maintain performance integrity after it is reassembled.
The present invention provides a modular radiation detector assembly and method characterized by a scintillator module containing a scintillator such as a scintillator crystal, and an electronics module containing a light sensing device such as a photomultiplier tube (PMT). The scintillator module and the electronics module are releaseably mechanically coupled, for example by means of mating threaded portions on each of the modules. The crystal and the PMT are optically coupled under a prescribed loading determined by the mechanical coupling. Preferably, a resilient optical interface such as a gel pad is pressed between an optical window of the scintillator module and window side of the electronics module when the modules are mechanically coupled together.
Thus, according to one aspect of the invention, a scintillator module is releaseably mechanically coupled to an electronics module.
According to another aspect of the invention, a radiation detector includes a scintillator module having a casing and a scintillation crystal within the casing, and an electronics module having a photodetector casing and a light sensing device within the photodetector casing, wherein the crystal and the light sensing device are optically coupled, and the crystal casing and the photodetector casing are releaseably mechanically coupled.
According to yet another aspect of the invention, a radiation detector includes a hermetically-sealed scintillator module and an electronics module, the modules being releaseably mechanically coupled via threaded portions on each of the modules. The threaded portion of the electronics module preferably is made of a softer material than the threaded portion of the scintillator module.
According to a further aspect of the invention, a radiation detector includes a scintillator module which is releaseably coupled to an electronics module. The electronics module includes a light sensing device, and a resilient biasing device which urges the light sensing device toward the scintillator module. The detector also includes an optical coupling device between the scintillator module and the electronics module, wherein the thickness of the optical coupling device affects the axial load placed on the light sensing device by the resilient biasing device.
According to a still further aspect of the invention, a radiation detector includes a scintillator module having a scintillator, an electronics module having a light sensing device, means for releaseably mechanically coupling the scintillator module and the electronics module, and means for optically coupling the crystal and the light sensing device.
According to another aspect of the invention, an electronics module for use in a radiation detector assembly includes a light sensing device and a photodetector casing circumscribing the light sensing device. The photodetector casing has a coupler at an inner end for mechanically coupling with a scintillator module. The light sensing device preferably is slidable within the casing under a biasing force.
According to still another aspect of the invention, a scintillation module for use in a radiation detector assembly includes a scintillator and a casing surrounding the scintillator, the casing housing a coupler at an optical window end therein for coupling with an electronics module.
According to yet another aspect of the invention, a method of using a radiation detector assembly includes the steps of mechanically coupling a scintillator casing of a scintillator module and a photodetector casing of an electronics module, and optically coupling a scintillator of the scintillator module and a light sensing device of the electronics module.
To the accomplishment of the foregoing and related ends, the invention comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.